The present invention refers to a chemical method of avoiding a rainbow effect which is caused by the layer of oxide produced upon the brightening of parts of aluminum or aluminum alloys, in which the parts are degreased or cleaned in a first process step and then brightened electrolytically/alkalinely in a second process step.
Aluminum parts, such as aluminum stampings or rolled sections are used, inter alia, in the hardware and lighting industries or, in particular, also in automobile manufacture, for instance for window mounting systems, or else as ornamented frames, ornamented moldings or the like. It is also known to use parts of aluminum or aluminum alloy which have a colored anodized surface.
Such aluminum or aluminum-alloy parts are, regardless of whether they are to be left in their natural color or are to be colored, first of all ground and, if necessary, then polished, degreased and brightened, the parts being treated for the brightening in an alkaline electrolyte which is subjected to DC current. It has been found that as a result of the layer of oxide which is produced upon the brightening of the parts, a rainbow effect in irridescent colors, which is extremely undesired, is produced. While the aforementioned effect is as a rule only slightly visible in the case of colorless (natural-color) anodized layers, it is frequently so strong, particularly in the case of colored anodized layers, that it is no longer acceptable. Treatment of the parts in chromic/phosphoric acid solutions has already been proposed. However, it has been found that such a treatment leads to a reduction in gloss and to an initial etching of the previously alkalinely brightened aluminum parts, so that such a treatment of the parts is unacceptable in practice.